Preheat oven to 425°. Line muffin pan with cupcake liners.

Combine butter and bittersweet chocolate in a small microwave-safe bowl. Microwave at HIGH for 45 seconds to 1 minute, stirring every 15 seconds, until melted and smooth. Cool to room temperature.

Combine flour, cocoa, baking powder, salt and baking soda in a large bowl; stir well with a whisk.

Combine granulated sugar, milk, vanilla, and eggs in a bowl; whip with a whisk until well combined.

Fold melted chocolate and liquid ingredients into flour mixture, stirring just until combined. Gently stir in chocolate chips. Divide batter evenly among 12 lined muffin cups.

Bake at 425° for 5 minutes. Reduce oven temperature to 375° for 10 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Do not overbake!